Metal oxide-modified ZnO/SiO <Subscript>2</Subscript> catalysts for cyclo-dehydrogenation of ethylenediamine with propyleneglycol to 2-methylpyrazine

Metal oxide-modified ZnO /SiO2 catalysts were studied for the cyclo-dehydrogenation of ethylenediamine with propyleneglycol to 2-methylpyrazine at 633 K. The ZnO/SiO₂ catalyst showed fairly good ethylenediamine conversion and quantitative propyleneglycol conversion with about 60 mol% of 2-methylpyrazine selectivity, which is due to the existence of large amount of unconverted intermediate, 2-methylpiperazine. Metal oxide (CuO, NiO, Co₃O₄)-modified ZnO/SiO₂ catalysts were prepared to facilitate the dehydrogenation of 2-methylpiperazine to 2-methylpyrazine. About 82 mol% of 2-methylpyrazine selectivity was achieved on CuO and Co₃O₄ modified ZnO/SiO₂ catalysts, with significant increases of pyrazine selectivity. The catalytic properties of the metal oxidemodified ZnO/SiO₂ catalysts, pretreated with hydrogen gas as in the cyclo-dehydrogenation, were compared using the well-known probe reaction, the dehydrogenation/ dehydration of cyclohexanol to cyclohexanone or phenol/cyclohexene. The selectivities of pyrazine in the cyclo-dehydrogenation on the metal oxide-modified ZnO/SiO₂ catalysts were correlated with the phenol selectivities of the probe reaction. It is proposed that the metallic site of catalyst is responsible for the formation of pyrazine from ethylenediamine dimerization. The improved 2-methylpyrazine yield on CuO/ZnO/SiO₂ catalyst was explained by the proper adjustment of catalytic properties, which could be differentiated by the phenol selectivity in the cyclohexanol probe reaction. Thus, the large enhancement of 2-methylpiperazine dehydrogenation to 2-methylpyrazine and the suppression of excess pyrazine formation are supposed to occur on the metallic Cu formed in situ during the reaction during the cyclo-dehydrogenation of ethylenediamine with propyleneglycol.     

四甲基吡嗪多元醇及其乙酸酯的合成

四甲基吡嗪经溴代和酯化反应得到2,3,5-三乙酰氧甲基-6-甲基吡嗪(4)和2,3,5,6-四乙酰氧甲基吡嗪(5);4和5分别经水解得2,3,5-三羟甲基-6-甲基吡嗪和2,3,5,6-四羟甲基吡嗪。其结构经UV,1H NMR,13C NMR,IR和MS表征。     

Synthesis of 2-Methylpyrazine on Ternary Oxide Catalysts

Synthesis of 2-methylpyrazine by cyclocondensation of ethylenediamine with 1,2-propylene glycol in the presence of zinc-molybdenum-chromium catalysts was studied.     

Electron-impact induced skeletal re-arrangement of 2-methoxy-3-methylpyrazine and 2-methylthio-3-methylpyrazine

The mass spectra of 2-methoxy-3-methylpyrazine (I), 2-methoxy-6-methylpyrazine (II), 2-methylthio-3-methylpyrazine (III) and 2-methylthio-6-methylpyrazine (IV), are given and the major fragmentation pathways discussed. The novel loss of H₂O from the molecular ion of I and the corresponding loss of H₂S from the molecular ion of III indicate that a skeletal rearrangement takes place in the molecular ion preceding the expulsion of H₂O and H₂S. Proposed mechanisms for this behavior are discussed with evidence being drawn from accurate mass measurement, metastable ions, and deuterium and carbon-13 labeling of the methoxy group. The absence of ions in the spectra of II and IV corresponding to the loss of H₂O and H₂S from these molecular ions clearly indicates that the position of the methyl group with respect to the methoxy group, or the methylthio group is in-timately involved in this mechanism.     

Synthesis of pyrazinyl compounds from glycerol and 1,2-propanediamine over Cu-TiO2 catalysts supported on γ-Al2O3

Cu-TiO₂ catalysts supported onγ-Al₂O₃ are prepared and used in glycerol cyclization with 1,2-propanediamine to produce pyrazinyl compounds including 6-hydroxymethyl-2-methylpyrazine,5-hydroxymethyl -2-methyIpyrazine,2,6-dimethylpyrazine and 2,5-dimethylpyrazine in a fixed-bed system.It is found that glycerol cylclization with 1,2-propanediamine gave a high total yield of pyrazinyl compounds（>80%） over Cu-Ti02/γ-Al₂O₃ catalyst,and cyclization was through the reactions between activated 1,2-propanediamine and the intermediates from glycerol dehydration and oxidation.In addition,the regioselectivity of the pyrazinyl compounds was mainly controlled by the steric hindrance of the substrates during the cyclization process.     

Binary Oxide Systems in Catalytic Synthesis of 2-Methylpyrazine from 1,2-Propylene Glycol and Ethylenediamine

A series of binary catalysts based on oxides of zinc and variable-valence metals were tested in synthesis of 2-methylpyrazine by catalytic dehydrocyclization of 1,2-propylene glycol with ethylenediamine.     

Coffee aroma—Statistical analysis of compositional data

Solid-phase microextraction in headspace mode coupled with gas chromatography-mass spectrometry was applied to the determination of volatile compounds in 30 commercially available coffee samples. In order to differentiate and characterize Arabica and Robusta coffee, six major volatile compounds (acetic acid, 2-methylpyrazine, furfural, 2-furfuryl alcohol, 2,6-dimethylpyrazine, 5-methylfurfural) were chosen as the most relevant markers. Cluster analysis and principal component analysis (PCA) were applied to the raw chromatographic data and data processed by centred logratio transformation.     

CuZnAl mixed-metal oxides prepared by a novel sol–gel route and the application for synthesis of 2- methylpyrazine

Metal (Cu, Zn, Al) nitrates and chlorides were used for preparing CuZnAl xerogels by a sol–gel route with propylene oxide as gelation initiator. The CuZnAl mixed-metal oxides were further obtained by thermal treatment the xerogels at 500 °C for 5 h in air. Thermal decomposition behavior of the CuZnAl xerogels, the microstructures and the reducibility of the calcined xerogels were investigated by thermogravimetry (TG), scanning electron microscope (SEM), powder X-ray diffraction (XRD), N₂ adsorption/desorption (BET, BJH) and temperature-programmed reduction (TPR) techniques. The catalytic activity in dehydrocyclization of ethylenediamine (ED) with 1, 2-propyleneglycol (PG) to 2-methylpyrazine (2-MP) was carried out at 380 °C. The results displayed that the CuZnAl mixed-metal oxides prepared using nitrates as the metal precursors had a higher metallic Cu dispersion and a superior low-temperature reducibility than those prepared by chlorides, which results in a higher catalytic activity for the synthesis of 2-methylpyrazine. Especially when the molar ratio of Cu/Zn/Al = 2:1:2, the catalyst using nitrates as the metal precursors improved the selectivity of 2-MP to 87.5%.     

气相氨氧化2-甲基吡嗪制备2-氰基吡嗪研究进展

综述了近年来国内外有关氮氧化2-甲基吡嗪合成2-氰基吡嗪反应的研究进展,讨论了催化剂及各种反应条件对活性的影响,分析了反应历程和机理。含钒和含钼氧化物催化剂具有较好的催化活性,原料中适量的氧气和氮气有利于反应活性的提高,2-甲基吡嗪转化率和2-氰基吡嗪选择性随着温度提高而增加,但温度过高使选择性降低,一般选取350～430℃。     

Closed-pore crystal capable of adsorbing CO2 onto isolated cavities generated by disorderly mixing of substituents on host skeleton

A single crystal adsorbent, [Rh(II)(2)(bza)(4)(2,3-empyz)](n) (2,3-empyz = 2-ethyl-3-methylpyrazine) (1), was synthesized by self-assembly reaction of a Rh(2) benzoate complex and substituted pyrazine linker. The compound consists of one-dimensional zigzag chains, which generated a closed-pore structure without channels. The cavities were statistically generated by the static disorder of substituents on pyrazine and are separated by long intervals within the crystal. The property of CO(2) absorption was characterized in this closed-pore system. The CO(2) inclusion structure was determined by single-crystal X-ray diffraction measurements. These studies suggest that CO(2) molecules were adsorbed and diffused in the nonporous crystal with the isolated cavities.     

Studies on pyrazines. 9 . The facile synthesis of pyrazine 1-oxides substituted at C-2 with cyano,methoxycarbonyl and carboxy groups

The peroxysulfuric acid oxidation of 2-cyanopyrazine ( 1 ) gave its 1-oxide 3 (18% yield) and pyrazinecarb-oxamide (8% yield), while that of methyl pyrazinecarboxylate ( 2 ) provided the 1- and 4-oxides in 15 and 7% yields, respectively. On the other hand, pyrazinecarboxylic acid 1-oxide ( 7 ) was prepared by condensation of 2-methylpyrazine 1-oxide ( 8 ) with benzaldehyde followed by oxidative cleavage (47% overall yield).     

空穴传输基团修饰的吡嗪铱配合物的合成及其发光性质

设计合成了空穴传输性咔唑基团修饰的吡嗪配体, 2,3-二(4-(9-咔唑基甲基)苯基)-5-甲基吡嗪(CzMPMP)及其铱配合物Ir(CzMPMP)2(acac)(acac: 乙酰丙酮). 用核磁共振(NMR)、质谱、元素分析等方法对其进行了表征, 并用紫外-可见吸收光谱、液相和固相光致发光光谱对其光学性质进行了研究. 通过与没有咔唑取代基的2,3-二苯基-5-甲基吡嗪铱配合物Ir(DPMP)2(acac)进行对照, 说明咔唑基团对配合物的光学性质有显著影响. 不含咔唑取代基的配合物Ir(DPMP)2(acac)的固态光致发光光谱中存在很强的激活双体(excimer)峰, 而Ir(CzMPMP)2(acac)的固态光致发光光谱中未发现有激活双体峰; 并且在溶液中, 含有咔唑取代基的配合物Ir(CzMPMP)2(acac)较Ir(DPMP)2(acac)的光致发光强度有显著提高. 这些结果说明咔唑基团的空间位阻可以有效阻止配合物成膜时产生激活双体, 从而消除发射峰红移现象并提高发光效率.     

Interaction-induced enhancement in the activity and selectivity of a titania-supported ammonium salt of a 12-molybdophosphoric acid catalyst during ammoxidation of 2-methylpyrazine

A titania-supported ammonium salt of 12-molybdophosphoric acid has been synthesized, and the salt-support interaction, which enhanced the reaction rate, has been correlated with the activity of the catalyst in the ammoxidation of 2-methylpyrazine.     

Influences of pore size on production of 2-methylpyrazine over bifunctional CuO/ZnO/meso-SiO2 catalysts

Controllable crystallite size of CuO in CuO/ZnO/meso-SiO₂ catalysts was successfully realized by impregnation method using mesoporous silica with different pore diameters as support. Characterization techniques such as N₂ adsorption/desorption, X-ray diffraction, H₂ temperature-programmed reduction, and potentiometric titration were employed to investigate the influences of pore size on textural properties, crystalline phases, reducibility, and acidity. Catalytic evaluation of synthesis of 2-methylpyrazine was carried out at 380 °C under atmospheric conditions. The best catalytic performance was achieved over catalyst CZ/S1 supported on the carrier with smallest pore size.     

Mixed valence isomers

The infrared spectroscopic observation of mixed valence isomers, the two alternate charge distributions of a mixed valence complex, is reported. Asymmetry induced by the 2-methylpyrazine bridging ligand was used to energetically differentiate the two states, and isotopic labeling of CO was used to spectroscopically observe the two states. Infrared line shape analysis was used to determine rate constants for electron transfer of 6.5 x 1011 s-1 and equilibrium constants of 2.2 for the mixed valence isomers.     

Synthesis of tetrasubstituted pyrazines and pyrazine N-oxides

An efficient synthesis of tetrasubstituted unsymmetrical pyrazines and their related pyrazine N-oxides has been developed from commercially available 2-chloro-3-methylpyrazine. The procedure and scope of these synthesis routes are described.     

钴(Ⅱ)催化2-乙基-3-甲基吡嗪绿色氧化方法

以过氧化叔丁醇为氧化剂,以钴(Ⅱ)与含N配体为催化体系催化氧化2-乙基-3-甲基吡嗪(EMP),提出一种具有放大应用前景的2-乙酰基-3-甲基吡嗪绿色制备方法.考察了过渡金属催化剂的种类、配体种类、溶剂、温度等反应条件对催化氧化过程的影响,在最优反应条件下EMP转化率可达58.8%, 2-乙酰基-3-甲基吡嗪(AMP)选择性92.2%.研究了该反应体系催化氧化EMP的反应机理,建立了该反应的拟均相反应动力学模型.在以上实验基础上,对该反应体系进行了放大实验研究,结果表明该新方法具有较好的工业放大前景,但反应温度的控制是放大过程的关键因素.     

Polarography of an acidic degradation product from cephalexin

2-Hydroxy-3-phenyl-6-methylpyrazine is identified as the product obtained by acidic degradation of cephalexin in the presence of formaldehyde. In 5M hydrochloric acid this product gives a well-defined reduction wave with a half-wave potential of -0.45 V vs. SCE. The wave is irreversible and diffusion-controlled. The diffusion current shows a linear relation with the cephalexin concentration and can be used for determination of cephalexin in plasma.     

Tuning the surface composition of novel metal vanadates and its effect on the catalytic performance

Tuning the surface composition of metal vanadates using different cations leads to the development of a new class of highly effective catalysts tested in the ammoxidation of 2-methylpyrazine. Especially, an enrichment of V in the near-surface region is beneficial for improved selectivity. With this approach, a knowledge based optimisation of the catalysts was possible for the first time, which indeed led to highly efficient novel LaVO(x) catalysts with a high yield of 2-cyanopyrazine (≥85%) and extremely high space-time-yields (ca. 525 g kg h(-1)).     

Synthesis of pyrazine acetals by the biased reaction of symmetric 2,5-bis(chloromethyl) or 2,3,5,6-tetrakis(chloromethyl) substituents on pyrazine ring with sodium alkoxides

2,5-Bis(chloromethyl)pyrazine reacted with sodium alkoxide to give unexpected 2-dialkoxymethyl-5-methylpyrazine along with normal substitution product, 2,5-bis(alkoxymethyl)pyrazine. The reaction of 2,3,5,6-tetrakis(chloromethyl)pyrazine with sodium alkoxide afforded similar results to yield 2,6-bis(dialkoxymethyl)-3,5-dimethylpyrazine along with other alkoxymethylpyrazines. The ratio of products depended on the solvent and alkoxide used. A general discussion of the mechanism of such a pyrazine acetal synthesis in the basic conditions is given.